Just for information, I generally agree with what you said here, except in this particular case rsyslog is not a generic data bus, and Fluentd and logstash are,

Just for information, I generally agree with what you said here, except in this particular case rsyslog is not a generic data bus, and Fluentd and logstash are, so they're useful on their own merit. They're just often used as merely log transports, which overlaps with rsyslog.